1. Field of the Invention
The present invention relates to optical gate array devices, and more particularly, to an optical gate array device having an array of optical gates for controlling the transmission of optical signals.
2. Description of the Related Art
With the recent increasing demand for broadband communication services, optical communication networks have become capable of carrying a large volume of data over long distances, and the development of high-speed large-capacity WDM (Wavelength Division Multiplex: wavelength division multiplexing technique for multiplexing different wavelengths of light to simultaneously transmit multiple signals over a single optical fiber) has been actively pursued.
Also, because of the rapid diffusion of the Internet and an increase in large-capacity content, optical communication networks capable of higher-speed, larger-capacity data transmission and having flexibility are demanded. In the circumstances, optical packet switching is attracting attention as a technology for configuring such optical communication networks.
With the optical packet switching technology, communication information is switched directly in the form of optical packets. Compared with conventional switching techniques in which optical signals are once converted to electrical signals, no restriction is imposed by the electronic processing speed, and since optical signals can be processed at a rate equivalent to the light propagation delay time, high-speed, large-capacity transmission can be achieved.
In the case of switching an optical signal on a packet-by-packet basis, a gate switch is used to switch the optical signal ON and OFF. There are two major types of gate switch for switching optical signals ON and OFF through electric control, namely, the type adapted to vary the absorption of light by utilizing an electro-absorption effect, and the type adapted to vary the gain of a semiconductor amplifier by means of a driving current supplied thereto.
An electro-absorption type gate switch has a drawback in that the loss is high even in the state of transmission. On the other hand, a semiconductor optical amplifier (SOA), which is a switch adapted to vary its gain by means of the driving current supplied thereto, not only functions as an optical gate for switching light ON and OFF but also has an amplifying function (when the gate is ON, light amplified thereby is output). Thus, SOA is currently watched as an optical device capable of high-speed switching with low loss of optical signal.
Further, SOA has a large extinction ratio between gate ON (open) and OFF (closed) states and is also capable of reducing optical loss by means of its amplifying mechanism. Moreover, since SOA is an optical device made of semiconductor, small-sized SOA can be fabricated at low cost by using semiconductor integration technology.
FIG. 18 shows a conventional arrangement for optical coupling between an SOA and an optical fiber. If light pumped inside the chip of an SOA 51 is reflected at its end face, unwanted oscillation is caused by the reflected light, deteriorating the characteristics of the SOA. It is therefore necessary that the end face of the SOA should have a low reflectance of −50 dB or less.
Accordingly, the end face of the SOA 51 is coated with an AR (Anti Reflection) coating (not shown), which is a non-reflective film. However, the AR coating alone is unable to satisfactorily reduce the return loss, and therefore, the SOA 51 is obliquely positioned such that the normal H perpendicular to the end face of the SOA 51 and an optical waveguide L within the SOA 51 form an angle of, for example, 7°.
Since the SOA 51 is positioned in this manner, light from an optical fiber 52a obliquely passes through the SOA 51 along the optical waveguide L toward an optical fiber 52b, and the light reflected at the end face of the chip propagates in a direction A shown in the figure (at an angle of 14° with respect to the optical waveguide L). Thus, the reflected light is prevented from returning back through the optical waveguide L, and therefore, does not interfere with the incoming light.
Let it be assumed that the refractive index of the light incidence-side medium is n1, that the incidence angle is θ1, that the refractive index of the light emergence-side medium is n2, and that the emergence angle is θ2. From Snell's law, n1·sin θ1=n2·sin θ2, and in the case where the refractive index n1 of the material of the SOA 51 is 3.2, then 3.2·sin 7°=1·sin θ2, because the incidence angle θ1 with respect to the end face is 7° and the refractive index n2 of air is 1. Consequently, the emergence angle θ2 is nearly equal to 22.7°, that is, light is output from the end face of the SOA 51 at the emergence angle 22.7°.
Thus, the light output from the end face of the SOA 51 at the emergence angle 22.7° is input to the optical fiber 52b. Since the SOA 51 is obliquely positioned, lenses 53a and 53b are used to achieve optical coupling between the SOA 51 and the respective optical fibers 52a and 52b. Specifically, the lens 53a optically couples the input-side optical fiber 52a with the SOA 51, and the lens 53b optically couples the output-side optical fiber 52b with the SOA 51.
FIG. 19 also shows a conventional arrangement for optical coupling between the SOA 51 and an optical fiber, wherein spherical lensed fibers 54a and 54b are used in conjunction with the SOA 51, by way of example. The distal end of each of the spherical lensed fibers 54a and 54b is formed into a spherical shape and serves as a lens, and therefore, the lenses 53a and 53b shown in FIG. 18 can be omitted.
As conventional techniques using SOA, a technique has been proposed in which a semiconductor optical amplifier is used in combination with an external resonator constituted by a fiber grating, and the fiber grating has a distal end formed into a spherical shape to be optically coupled with the light emergence end face of the semiconductor optical amplifier coated with a low-reflection film (e.g., Unexamined Japanese Patent Publication No. 2000-236138 (paragraph nos. [0045] to [0054], FIG. 1)).
FIGS. 20 and 21 each illustrate the optical coupling between an SOA array and an optical fiber array. The figures individually show only one side of the arrangement, with an input-side optical fiber array and an input-side lens array omitted. In FIG. 20, an optical fiber array 64, which is an array of optical fibers 64a to 64d, is optically coupled with an SOA array 61, which is an array of SOAs 61a to 61d, through a lens array 62, which is an array of lenses 62a to 62d. In FIG. 21, a spherical lensed fiber array 65, which is an array of spherical lensed fibers 65a to 65d, is optically coupled with the SOA array 61.
In either of the arrangements shown in FIGS. 20 and 21, when optically coupling the SOA array and the optical fiber array, it is necessary that the pitch P1 (distance between the optical waveguides of adjacent SOAs) of the SOA array should be equal to the pitch P2 (distance between the centers of the cores of adjacent optical fibers) of the optical fiber array.
When manufacturing SOA arrays, on the other hand, the pitch P1 of the SOA array should preferably be reduced as small as possible, in order to increase the number of SOAs mounted per unit area and thereby heighten the degree of integration. However, in conventional SOA arrays, SOAs should not be arrayed with a pitch smaller than the diameter of the optical fiber, giving rise to the problem that the degree of integration of SOA arrays cannot be improved.
FIG. 22 illustrates the problem associated with the conventional optical coupling arrangements. In order to mount more SOAs per unit area of a wafer (thin substrate of semiconductor used for the manufacture of IC chips), SOAs need to be arrayed with a narrower pitch.
In the conventional optical coupling arrangements, however, the pitch P1 of the SOA array must be equal to the pitch P2 of the optical fiber array (P1=P2), in order for the optical coupling to be achieved between the SOA array and the optical fiber array. Thus, as seen from the figure, the narrowest allowable pitch of the SOA array is equal to the pitch with which optical fibers are arrayed in contact with each other, namely, the pitch equal to the diameter of the optical fiber.
Specifically, ordinary optical fibers have a diameter of 125 μm, and therefore, the pitch of the SOA array should be 125 μm at the smallest. Accordingly, even though more SOAs can be mounted on the wafer, the conventional optical coupling arrangements do not permit SOAs to be arrayed with a pitch smaller than 125 μm corresponding to the diameter of optical fibers, posing a problem that the degree of integration of SOA arrays cannot be improved (if the pitch of the SOA array is set smaller than the diameter 125 μm of optical fibers, then the optical coupling between the SOAs and the optical fibers cannot be achieved).
Further, the SOA has a beam spot size (the radius of a light beam passing through the optical waveguide of the SOA) smaller than that of the optical fiber. A problem therefore arises in that the conventional optical coupling arrangements are poor in optical coupling efficiency.